This invention relates to a removable adhesive composition, more particularly to a removable adhesive composition having an acrylate or methacrylate ester series copolymer as a main component and having a pressure-sensitive adhesion performance at ordinary temperature.
As important properties required for an adhesive to be used in forming an adhesive tape or sheet which can be pressure sensitively adhered to an adherend at ordinary temperature and under ordinary conditions, there may be mentioned such properties as primary tack, adhesive force and cohesive force. It has also been known that an excellent adhesive may not be obtained unless the balance of these properties is maintained favorably.
Here, primary tack means the primary self-adhesive force which may simultaneously be exhibited at the moment when an adhesive comes into contact with an adherend. The higher the value of this tack is, the more favorable the adhesive may be. In order to allow an adhesive to exhibit high primary tack favorably, the adhesive is required to give increased areas which are capable of being in contact with an adherend as widely as possible within a shorter time and at lower pressure.
Adhesive force is a property which may be observed when an adhesive tape or an adhesive sheet coated with an adhesive is being removed from an adherend in at least a few seconds after the adhesive was in contact with the adherend. In general, the greater the contact area of an adhesive with an adherend is, the greater the adhesive force may be. The adhesive force may also change on the viscoelastic nature of the adhesive.
Cohesive force means the cohesiveness that an adhesive itself has, which is required to prevent the adhesive from remaining on the surface of an adherend with legginesses when the adhesive is removed therefrom. In many cases, the cohesive force of an adhesive can be determined by measuring the viscosity of the adhesive. As viscosity which may satisfy the condition not to cause the legginesses, there has been known a viscosity of 1.times.10.sup.6 poise or more at ordinary temperature when removed at usual removing speed.
On the other hand, the adhesive force of an adhesive to a substrate is required to be always greater than that to an adherend so that the adhesive may not remain on the surface of the adherend. For this purpose, it has been often practiced that an adhesive is designed to exhibit a state that it roots in the substrate, i.e., the so-called anchor effect. It has been also practiced that a molecule constituting a substrate is allowed to be chemically bonded to a molecule constituting an adhesive.
As described above, there may be mentioned four points which are required for a removable adhesive capable of bonding to an adherend more strongly and removing therefrom without leaving any remanent adhesive; namely, high primary tack (primary self-adhesiveness); adhesive force to maintain an adhesive sheet in a state being bonded to an adherend as necessary; cohesive force that an adhesive may neither cause legginess nor remain on the surface of the adherend when removed; and that the adhesive force of the adhesive to a substrate is sufficiently greater than that to an adherend.
In order to impart to an adhesive such properties as primary tack, adhesiveness and removability at the same time, it is necessary for the adhesive to satisfy the above-described requisites with a favorable balance.
Japanese Provisional Patent Publication No. 2736/1975 discloses a body having structure wherein adhesive micro-balls comprising a copolymer formed by using water-emulsifiable alkyl acrylate esters and water-soluble ionic vinyl monomers such as sodium acrylate are bonded on a substrate with a binder in such a manner that the micro-balls may partially protrude from the surface of the binder. In this Publication, it is disclosed that by use of the adhesive micro-balls a series of excellent performance for the tacking, adhesion and removal, can be made. It is also described therein that the binder and the adhesive micro-balls need not be chemically bonded. As to the reason therefor, it is explained that transfer of the adhesive to an adherend may be effectively prevented by the physical socket effect.
Furthermore, such a type of adhesive micro-balls is also disclosed in U.S. Pat. No. 3,691,140 issued on Sep. 12, 1972, wherein the basic concept of the adhesive composition is the same as mentioned above and a water-soluble ionic monomer is employed. This ionic monomer is incorporated in a copolymer and plays a role of preventing particles from agglomeration during the course of suspension polymerization in water; on the other hand, it becomes a component which gives unfavorable influence on the adhesiveness of the adhesive micro-balls produced. Thus, it is difficult to obtain an adhesive which has favorable primary tack in the case where the water-soluble ionic monomer is employed.